SACRAMENTO — California's Democratic governor signed a law Tuesday requiring presidential candidates to release their tax returns to appear on the state's primary ballot, a move aimed squarely at Republican President Donald Trump.

But even if the law withstands a likely legal challenge, Trump could avoid the requirements by choosing not to compete in California's primary. With no credible GOP challenger at this point, he likely won't need California's delegates to win the Republican nomination.

While aimed at Trump, the law also applies to candidates for governor. Newsom said California's status as one of the world's largest economies gives it "a special responsibility" to require tax returns from its prospective elected officials.

"These are extraordinary times and states have a legal and moral duty to do everything in their power to ensure leaders seeking the highest offices meet minimal standards, and to restore public confidence," Newsom wrote in his signing statement.

The Trump campaign called the bill "unconstitutional," saying there were good reasons why California's former Democratic Gov. Jerry Brown vetoed a similar proposal last year.

"What's next, five years of health records?" said Tim Murtaugh, communications director for Trump's campaign.

The courts will likely have the final say. The bill's author, Democratic state Sen. Mike McGuire, said lawmakers made sure the law only applies to the state's primary ballot because the state Constitution says the state Legislature does not control access to the general election ballot.

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Newsom's message to state lawmakers on Tuesday said the law is constitutional because "the United States Constitution grants states the authority to determine how their electors are chosen."

But Murtaugh said the law violates First Amendment right of association "since California can't tell political parties which candidates their members can or cannot vote for in a primary election."

While states have authority over how candidates can access the ballot, the U.S. Constitution lays out a limited set of qualifications someone needs to meet to run for president, said Rick Hasen, a professor specializing in election law at the University of California-Irvine School of Law. Those qualifications include the requirement that presidential candidates be over age 35.

The U.S. Supreme court has previously stopped state efforts to add requirements on congressional candidates through ballot access rules.

New York has passed a law giving congressional committees access to Trump's state tax returns. But efforts to pry loose his tax returns have floundered in other states. California's first attempt to do so failed in 2017 when then-Gov. Jerry Brown, a Democrat, vetoed the law, raising questions about its constitutionality and where it would lead next.

The major Democratic 2020 contenders have already released tax returns for roughly the past decade. Trump has bucked decades of precedent by refusing to release his. Tax returns show income, charitable giving and business dealings, all of which Democratic state lawmakers say voters are entitled to know about.

California's new law will require candidates to submit tax returns for the most recent five years to California's Secretary of State at least 98 days before the primary. They will then be posed online for the public to view, with certain personal information redacted.

California is holding next year's primary on March 3, known as Super Tuesday because the high number of state's with nominating contests that day.

Democratic Sen. Mike McGuire of Healdsburg said it would be "inconsistent" with past practice for Trump to forego the primary ballot and "ignore the most popular and vote-rich state in the nation."

Republican Party of California chairwoman Jessica Millan Patterson said Newsom signing the law shows Democratic leaders in the state continue "to put partisan politics first," urging Democrats to instead join Republicans "in seeking ways to reduce the cost of living, help our schools and make our streets safer."